Laplace Pressure Difference Enhances Droplet Coalescence Jumping on Superhydrophobic Structures

Abstract

Coalescence-induced droplet jumping has great prospects in many applications. Nevertheless, the applications are vastly limited by a low jumping velocity. Conventional methods to enhance the droplet coalescence jumping velocity are enabled by protruding structures with superhydrophobic surfaces. However, the jumping velocity improvement is limited by the height of protruding structures. Here, we present rationally designed limitation structures with superhydrophobic surfaces to achieve a dimensionless jumping velocity, V j * ≈ 0.64. The mechanism of enhancing the jumping velocity is demonstrated through the study of numerical simulations and geometric parameters of limitation structures, providing guidelines for optimized structures. Experimental and numerical results indicate that the mechanism consists of the combined action of the velocity vectors' redirection and the Laplace pressure difference within deformed droplets trapped in limitation structures. On the basis of previous research on the mechanisms of protruding structures and our study, we successfully exploited those mechanisms to further improve the jumping velocity by combining the limitation structure with the protruding structure. Experimentally, we attained a dimensionless jumping velocity of V j * ≈ 0.74 with an energy conversion efficiency of η ≈ 48%, breaking the jumping velocity limit. This work not only demonstrates a new mechanism for achieving a high jumping velocity and energy conversion efficiency but also sheds lights on the effect of limitation structures on coalescence hydrodynamics and elucidates a method to further enhance the jumping velocity based on protruding structures.

Superhydrophobic Antifrosting 7075 Aluminum Alloy Surface with Stable Cassie–Baxter State Fabricated through Direct Laser Interference Lithography and Hydrothermal Treatment

“…The Cassie–Baxter state is more stable when the critical Laplace pressure at the wetting transition is higher. The critical Laplace pressures during the wetting transition of the majority of reported single-scale structures or simple micro/nanostructures, however, are less than or close to 500 Pa, and the values such as 290.8 Pa, 460 Pa, 400 Pa, and ∼500 Pa are listed.…”

Exploration of Sweeping Effect: Droplet Coalescence Jumping of a Rolling and Static Droplet

“…As shown in Figure , a high-speed visualization platform (20,000 fps) was established to record the coalescence process and measure jumping characteristics. The superhydrophobic surface was fabricated on a slide surface using commercial superhydrophobic nanocoating reported in our previous works, , allowing us to ignore the effects of surface features of substrates. The static contact angle of a water droplet size of 0.4 μL is 163 ± 2°, and the rolling contact angle is less than 3 ± 1°, indicating that the rolling movement of the droplet can be easily achieved on the superhydrophobic surface.…”

Coalescence-Induced Droplet Jumping on Superhydrophobic Surfaces with Annular Wedge-Shaped Micropillar Arrays

“…Coalescence-induced droplet jumping on superhydrophobic surfaces has attracted much attention due to its great potential for applications in enhanced condensation, − water harvesting, , thermal management of electronic devices, − and microfluidics. − In 2009, Boreyko et al discovered the droplet jumping phenomenon on superhydrophobic surfaces with carbon nanotubes and silicon micropillars, measured the jumping velocities of droplets with radii ranging from 8 to 160 μm, and proposed that the jumping velocities follow the inertia-capillary scaling law. Coalescence-induced droplet jumping is triggered by the interaction between the droplet and the surface and the conversion of a portion of the released surface energy during droplet coalescence into the kinetic energy of jumping.…”